In a cellular communication system, a geographical region is divided into a number of cells each which are served by base stations. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link from the base station of the cell within which the mobile station is situated.
A typical cellular communication system extends coverage over an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as the uplink, and communication from a base station to a mobile station is known as the downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Internet or the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD). In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and in the same time intervals. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a 3rd generation cellular communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.
The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate Node Bs. It further provides the interface between the RAN and the core network. An RNC and associated Node Bs is known as a Radio Network Subsystem (RNS).
The interface between the RNC and the Node B is known as the Iub interface. As much of the functionality associated with communicating over the air interface is implemented in the RNC, and as the air interface traffic data is routed to the RNC, a significant amount of data is transferred across the Iub interface. Accordingly, high capacity communication links between RNCs and Node Bs are required.
Specifically for the 3GPP UTRAN (UMTS Terrestrial Radio Access Network), packet data for downlink transmission is buffered at the RNC and transmissions are also scheduled at the RNC. The scheduler is part of the RRC (RRC—Radio Resource Control) protocol. Typically, scheduling for each cell is performed autonomously and without direct communication between the different schedulers. Scheduled data is passed from the RNC to the Node B over the Iub interface. Uplink packet data is also scheduled by the RNC and traverses the Iub in the opposite direction.
In most cellular communication systems, the cost of the communication links between the RNCs and the Node Bs is one of the most significant operating and roll out costs associated with a cellular communication system. Therefore, it is desirable to use any communication capacity of the Iub communication links as efficiently as possible to reduce back-haul costs. One approach for reducing back-haul costs is to share an Iub communication link between different cells, cell sectors or base stations. For example, two or more cells may share a single E1 leased line offering 2 Mb/s in each direction.
In some deployments where the Iub communication links are shared these may be dimensioned to support less than the aggregate air interface capacity of the subtended cells. For example, three cells could share a single E1 leased line offering 2 Mb/s in each direction (a typical 3GPP cell has a capacity of the order of 1 Mb/s in each direction). In this case, in a simple equal sharing of the Iub, each cell is allocated a third of the capacity of the E1 link resulting in each cell having a capacity of ⅔ Mb/s in each direction.
Although such an approach may provide cost savings, it may also result in reduced performance of the cellular communication system. For example, a highly loaded cell may require 1 Mb/s in each direction to support the current traffic load. As this is not available due to the restriction of the shared Iub connection, the effective capacity of the cell is reduced thereby resulting in a reduction of the capacity of the cellular communication system as a whole.
As another example, the sharing of the communication link is very inflexible and may result in the loading of one cell being restricted by the allocated capacity of the shared link while another cell is not fully using the capacity available to it. Thus, a situation may result where the loading of a cell is limited by an Iub communication link having spare capacity.
An improved system for scheduling data from a network element, such as an RNC, to base station(s) serving a plurality of cell sectors would be advantageous and in particular a scheduling approach allowing for increased flexibility, increased performance, low complexity and/or an improved utilisation of a shared communication link would be advantageous.